Electrochemical reactions that involve the transfer of charge at an electrode-solution interface are examples of a general class of reactions referred to as heterogeneous process, i.e. a chemical change occurs in which two or more reactions take place simultaneously. The kinetics of heterogeneous reactions are normally determined by a sequence of steps involving both transport through a solution phase and transfer of charge at the interface of the solution-electrode. This overall electrochemical reaction involves two step groupings, which are commonly referred to as the activation process and the mass transport process and the rates of the individual processes are somewhat time dependent.
These processes have been the subject of extensive study at the laboratory level for many years and applications of the underlying science have been made to anti-corrosion systems. The latter applications are board in scope, for example involving devices designed for the protection of large structures ranging from public water supply tanks to sports stadiums. Generally, the essential parts of such systems will include a source of direct current and voltage, a "counter electrode" for applying this current into any of a broad range of electrolytes, a "working electrode" which is the structure to be protected and a "reference electrode". If voltage and current conditions developed at the source are correct, an interface or boundary between the working electrode and electrolyte will be developed functioning to restrains the occurrence of chemical reaction which would otherwise result in corrosive activity. Thus, it is necessary to find essentially the precise conditions at this interface or boundary. The technique generally employed for this task is to utilize the noted reference electrode to sense the potential across the interface boundary and also sense the effect of the applied potential at the counter electrode. Generally, the current-voltage relationship at the working electrode is very non-linear. Thus, the amount of current required to develop necessary potential as evidenced at the reference electrode generally is determined for each of the large installations in somewhat experimental fashion, however, such experimental techniques are well known in the art and readily accomplished.
The determination of true potential at the working electrode through the use of a reference electrode, in generally terms, must take account of current flowing through the resistance of the electrolyte to that electrode, which develops an "IR drop" considered in a broad sense to exist between the working electrode and the reference electrode. That "IR drop", in effect, is added to the true potential and to a degree, serves to mask it. Thus, to find the true potential and accurately control the corrosion retarding system, a technique for canceling out this IR drop effect is required. A more refined analysis of the "IR drop" considers that a somewhat varying current density, j, evolves intermediate the counter electrode and the working electrode. The electrolyte, i.e. often concrete, will exhibit a complex, varying resistivity, .rho.. Compensation therefore must accommodate for the evolved vector quantity E=j.rho.. The potential is the (line integral) v=.intg.j.rho..multidot.ds; s being total path between the electrodes. Note that j is proportional to the impressed current, i, such that when current, i, is zero, j is zero and the noted integral value becomes zero, true potential being that voltage remaining when the integral goes to zero.
For the most part, corrosion control practitioners have employed a technique wherein the current from the counter electrode is momentarily interrupted such that it drops to zero and the corresponding masking IR drop is reduced to essentially a zero value. True potential then is measured quickly and stored. Such period of current shut-down must be limtied to a practical extent or the corrosion process will recommence upon any such removal of protective potential. The general practice has been to carry out this true potential evaluation about once each second. typically, a feedback form of control is employed in conjunction with a preset reference for purposes of maintaining predetermined true potential. While these techniques are relatively widely utilized, a need has arisen in the industry for higher levels of accuracy in the control systems as well as a broader rang of measurement parameters, i.e. current and voltages developed by the power supply for application to the counter electrode. Particularly, it has become important to provide very accurate measurements of the current supplied by the systems in recognition of the mass transport process law wherein the amount of reaction is proportional to charge transfer or average current multipled by time. Because for typical installations that current is fluctuating evaluations thereof have not been accurate. Further, the practioners have expressed a desire for a control over the voltage applied to the counter electrode. In effect, a greater degree of flexibility is required such that, under unusual circumstances which may occur where voltage excursions can cause extensive damage, i.e. internally coated water tanks and other installations, some form of over-voltage protection may be made available and a form of overriding control based upon current values is desirable.